Solar fired combined cycle with supercritical turbine

ABSTRACT

Mechanical work for electric power generation is obtained from thermal energy in a plant arranged for introduction of solar energy, available intermittently, by reflecting and concentrating solar radiation to directly heat a flow medium such as the exhaust gas from a combustion turbine directed into a steam generating boiler/evaporator. Steam generators and staged turbines recover and extract energy optimally at particular temperature, pressure and flow parameters in a closed thermodynamic cycle. Solar energy that is available intermittently is injected into the cycle to elevate the energy of the flow medium, in particular to produce supercritical steam. A steam turbine optimized for expanding supercritical steam is deployed during periods of available solar radiation by a controllable clutch and other switching and valve arrangements. The exhaust from the supercritical steam turbine can be coupled to downstream staged turbines optimized for successively lower pressures and higher flow rates.

FIELD OF THE INVENTION

This invention provides methods and apparatus wherein concentrated solarenergy is used, when sunlight is available, to heat the exhaust gas froma gas turbine to temperatures high enough for supercritical steamproduction in a heat recovery steam generator. The additionalsupercritical steam is expanded in a steam turbine for electric powergeneration at the highest possible thermal efficiency.

BACKGROUND OF THE INVENTION

Like many renewable power generation technologies, Concentrated SolarPower (CSP) suffers from intermittence and unpredictability, which canrender the technology difficult to justify and to deploy efficiently formeeting a constant demand for power. There are possible means toalleviate the discontinuous nature of energy supplied by an intermittentsource, such as to provide thermal energy storage facilities, but thatcan add to plant size and cost appreciably. Another remedy is tosupplement the solar thermal energy input by an on-demand energy source,for example by burning a fossil fuel when solar radiation isunavailable, but that runs against the purpose of deploying renewableenergy technologies in the first place. A current industry practice isto dispatch renewable power when it is available and to bring fossilfuel fired generation capacity on line (primarily Gas Turbine (GT) basedsimple or combined cycle power plants in spinning or non-spinningreserve mode) when renewable power sources are not available but thereis still a power demand to meet (e.g., at night, during periods of cloudcover, no wind, etc.).

A known solar-fossil hybrid technique uses solar thermal as asupplementary energy input. A well-known example is the Integrated SolarCombined Cycle (ISCC) power plant. In this concept, high pressure orintermediate pressure feed water from a Heat Recovery Steam Generator(HRSG) is sent to a CSP boiler (e.g., on a solar tower). The steam thatis generated is sent back to the HRSG. Another example is using a solartower or a parabolic trough heat collection system to heat feed water orto supplement the coal-fired boiler steam in a Rankine steam turbinepower plant. In all these concepts, solar thermal energy accounts foronly a small fraction of the total generation capacity (for example 10%)at a considerable expense and complexity. A CSP plant may have a largeplant footprint, where hundreds or thousands of heliostats or parabolictroughs are distributed across a solar field that may encompass hundredsof acres. The plant cost is further increased by the need for equipmentto handle complex integration issues such as a switching and controlphilosophy during solar thermal start and shutdown, among otherchallenges.

All existing ISCC concepts include making steam in a solar-powered“boiler.” Herein, the term boiler should be understood as a generic heattransfer system that converts water to steam. The most proven CSPtechnology deployed in ISCC applications is the parabolic trough, whichcomprises units with cylindrical parabolic reflectors having 4 to 5 mmthick silvered-glass mirrors. Solar energy concentrated by the trough istransferred to a synthetic-oil based Heat Transfer Fluid (HTF) flowingin a glass tube located at the focal point of the trough. For 1 MWthermal capacity, one needs a solar field of 4 to 5 acres containingrows of solar troughs with HTF tubes connected in loops. Hot HTF flowsinto a steam generator where pressurized feed water is converted tosteam. A main drawback of such solar trough technology is that themaximum achievable steam temperature is limited to about 745° F. by HTFavailability.

A recent CSP technology is the solar tower, which can generate highpressure steam at temperatures commensurate with modern steam turbinetechnology (i.e., 1,000° F. or higher). A solar tower system comprises atall tower (several hundred feet high) with a boiler on top thatreceives concentrated solar radiation from a field of heliostats, whichare dual-axis-tracking mirrors. In this form of boiler, high-temperaturesaturated or superheated steam is generated directly without anintermediate heat transfer medium.

The thermodynamic principle governing the optimal combination of solarand thermal steam generation in an ISCC is a direct manifestation of thesecond law of thermodynamics. Specifically, it is the principle ofmaximum exergy. Exergy (also known as availability) is a readilycalculable fluid property that translates the Kelvin-Planck statement ofthe second law into practical engineering calculations. In order tounderstand this concept within the context of a solar-thermal hybridpower plant, specifically ISCC, the reader is referred to FIG. 1.

As shown in FIG. 1, the interaction between the Gas Turbine CombinedCycle (GTCC) and the CSP plants can be simplified into two water/steamstreams:

-   -   Boiler feed water from the HRSG to the solar steam generator at        pressure and temperature, PFW and TFW, respectively; and,    -   Saturated or superheated steam from the solar steam generator to        the HRSG at pressure and temperature, PSTM and TSTM,        respectively.

It can be shown that an optimal ISCC design, as dictated by the exergymaximization principle, can be based on high pressure steam generationin the CSP plant. (See S. C. Gülen, 2013, “Second Law Analysis ofIntegrated Solar Combined Cycle Power Plants,” GT2014-26156, to bepresented in ASME IGTI Turbo 2014, Düsseldorf, Germany, Jun. 16-20,2014.) In order to appreciate this principle, consider that

-   -   1. The total energy input from the CSP plant to the GTCC plant,        Q_IN, is a fraction of the solar power incident normally on the        solar field when in sunlight.    -   2. The maximum additional power that can be generated from Q_IN        is equal to that which can be generated in a hypothetical Carnot        engine operating between the same hot and cold temperature        reservoirs.        -   a. This maximum (hypothetical) power is denoted by E_IN.        -   b. Actual additional power generated is a fraction of E_IN,            i.e., W_ADD=EFF2×E_IN, where EFF2 is the conversion            effectiveness (to be interpreted as a second law            efficiency).    -   3. The low temperature reservoir is at the existing ambient        temperature, TAMB.    -   4. The high temperature reservoir is at the Mean-Effective Heat        Addition Temperature, METH, which is the ratio of water and        steam enthalpy and entropy differences at the boundary between        the CSP and the HRSG in FIG. 1.    -   5. E_IN is a function of METH; the higher is METH, the higher is        E_IN.    -   6. EFF2 is also a function of METH; similarly, the higher is        METH, the higher is EFF2.    -   7. Thus, the higher is METH, the higher is W_ADD.

The current invention accomplishes its objects via increasing themean-effective heat addition temperature, METH, much beyond that ispossible with the existing technology shown in FIG. 1. This will beapparent in the paragraphs below when the details of exemplaryembodiments are disclosed.

A mechanism for increased CSP contribution is increasing high pressureor throttle steam flow (also referred to as the main steam flow) throughthe Steam Turbine (ST). In order to accomplish this, high pressure feedwater from the high pressure economizer is sent to the CSP plant steamgenerator and the high pressure steam is returned to the HRSG at thehigh pressure evaporator exit.

For a ST with a fixed volume swallowing capability, this is accompaniedby an increase in throttle pressure, which can go up to about 2,400 to2,500 psig. Higher throttle pressures can force the steam generationpressure in the HRSG high pressure evaporator to supercritical (above3,200 psia), which requires a once-through design (with no separatesteam drum). This is one of the two limiting factors in solar poweraugmentation; the other is the increase in the condenser pressure withincreasing low pressure turbine exhaust steam flow (a typical alarmlimit is 5 inches of mercury). This is a particularly critical item forplants with air-cooled condensers, which are very expensive in terms ofcapital expenditure and parasitic power consumption (power consumed bylarge fans driving the airflow through the condenser cells).

While the condenser pressure limitation can be overcome (e.g., by addingextra air cooled condenser cells to be deployed when low pressureexhaust flow is increased), the high pressure throttle pressure limit isnot so straightforward to accommodate. It requires the following systemdesign features:

-   -   1. Enough additional energy to sustain supercritical steam        generation (supplementary or duct firing, solar steam        augmentation);    -   2. Evaporator (boiler) section suitable to subcritical and        supercritical steam production (once-through Benson design);    -   3. Thick-walled balance of plant and steam turbine components to        withstand very high pressures (well above 3,000 psia) at high        temperatures (as high as 1,112° F. or even higher): valves,        pipes, turbine casings (costly alloys); and    -   4. An efficient steam turbine high pressure section at        (relatively) low volumetric flows (reaction design) driven by        high steam density.

One object of the current invention to provide solutions for the problempresented by the last item in that list.

SUMMARY OF THE INVENTION

It is an object of the current invention to provide a different way tointroduce solar energy into the bottoming cycle of a GTCC plant, so asto accommodate the intermittent availability of solar energy whileallowing the elements of the plant to be configured primarily foroperation in steady state conditions as opposed to very different levelsdepending on whether solar energy is available or not. Another object isto generate power with high solar conversion effectiveness by operatingpower conversion turbines and other apparatus at pressure, temperatureand flow rates that are optimized for performance whether solar energyis available or not. In particular,

-   -   1. Solar energy is added to the GTCC plant directly at the HRSG        (instead of at a receiver on a solar tower in a collector field,        at a substantial distance from the GTCC); and,    -   2. Solar energy is added to the GT exhaust gas (instead of at a        water/steam flowpath in the steam generator inside the solar        tower receiver).

Using solar radiation in a solar concentrator located between the gasturbine exhaust and the HRSG inlet, gas turbine exhaust gas (between1,100° F. and 1,200° F. for F class advanced machines) can be heated upto 1,600° F. (870° C.), which is a typical upper limit for duct-firedHRSGs. Duct firing (also known as supplementary firing) is a widelyadopted method for hot day power augmentation when gas turbines rapidlylose output due to lower compressor airflow at a time when power demandis high (especially in the U.S. with widespread use of air-conditioningin residential and industrial areas).

At this level of gas temperature, supercritical steam at very highpressure and temperature can be generated in a once-through boilersection in the HRSG (e.g., at least 3,750 psia, 1,112° F.). Thesupercritical steam from the HRSG is expanded in a steam turbine, whichis connected to a standard GTCC steam turbine via a SSS(Synchro-Self-Shifting) clutch and, if necessary, a gearbox or torqueconverter.

Supercritical steam turbine technology is known in coal-fired powerplant technology to generate electric power at high efficiency usingturbine structures and mechanisms that are optimized for hightemperature and pressure. In general, the “critical” point in steam is alevel of high temperature and pressure at which the separate gas andliquid phases of water merge into one continuous phase.

Deployment of solar energy in the manner proposed herein provides asurprising improvement in efficiency compared to current technology. Forexample,

-   -   1. Solar conversion efficiency is 5+ percentage points higher        (incremental steam turbine power output divided by solar thermal        input);    -   2. The solar fraction (SF) of power input to the plant can be        increased substantially, for example to up to nearly 25% (the        fraction of solar thermal input to the total fuel energy input);    -   3. Higher combined cycle efficiency (total power output divided        by fuel lower heating value (LHV) input) by about 3 percentage        points; can be as high as 70% net (85° F.-45% “hot” day basis        with and air-cooled condenser at full load).

The foregoing objectives are achieved in an apparatus for generatingmechanical work from thermal energy, primarily for coupling torque to anelectric generator. The apparatus includes at least a first powergeneration component operable to produce at least one flow medium withan elevated energy state from which energy can be extracted, and atleast one energy extraction component operable to extract mechanicalwork from the flow medium, extraction of such energy reducing the flowmedium to a less elevated energy state. A flowpath is defined for theflow medium from the first power generation component to the energyextraction component. When solar radiation is available, a solarconcentrator directs solar heat energy into the flow medium along theflowpath, thereby increasing the elevated energy state of the flowmedium as a function of the extent of solar energy that is available.

The first power generation component, such as a gas combustion turbine,is operated whether the solar radiation is available or not. The gasturbine is arranged to produce steam at elevated temperature andpressure. The steam turbine is coupled to at least one energy extractioncomponent such that mechanical work is extracted in conjunction withexpansion and cooling of the steam.

In order to achieve an acceptable efficiency, the energy extractioncomponent is sized and configured for optimal operation at particularflow, pressure and temperature characteristics. During availability ofsolar radiation, the additional thermal energy that is available wouldelevate the temperature and pressure out of the ranges in which theenergy extraction component operates most efficiently. It is an aspectof the invention that the solar radiation, when available, is used toproduce supercritical steam, that is steam at a sufficiently hightemperature and pressure that the gas (steam) and liquid (water) phasesof the flow merge into one phase. The supercritical steam is expanded ina turbine optimized for the supercritical steam (high pressure andtemperature with relatively low volume flow rate as compared to otherturbines in the plant). The exhaust from the supercritical high pressuresteam turbine can be coupled to further turbines staged for successivelylower pressures and temperatures and higher volume flow rates as thesteam is expanded. The supercritical high pressure turbine is deployedonly when solar radiation is available. Controllable mechanical shaftcouplings and flow couplings permit the supercritical high pressureturbine to be coupled when needed and decoupled in the absence of solarradiation sufficient to operate the supercritical high pressure turbine.

According to one aspect, the foregoing provisions are part of anelectric power generation system having at least one steam generatorcoupled in a recirculating flow path to supply a production flow ofsteam at nominal production pressure and temperature to a main steamturbine, the main steam turbine being mechanically coupled to anelectric generator for generation of electric power by extracting energyfrom expansion of the production flow of steam. At least one additionalsteam turbine is coupled to the recirculating flow path and operated ata second production pressure and temperature that are one of a higherenergy state than the nominal production pressure and temperature. Asolar collector is configured to apply concentrated solar radiation toat least one zone along the recirculating flow path during periods ofsufficient available solar energy, the solar collector increasing apressure and temperature at said zone to the second production pressureand temperature. A controller coupled to requisite pressure, temperatureand flow sensors assesses the condition of steam from the HRSG andoperates at least one valve and/or mechanical coupling for selectivelydeploying the additional steam generator as a function of theavailability of solar radiation.

The additional steam turbine, such as the supercritical steam turbine,can be mechanically coupled to contribute torque to turn the electricgenerator, through at least one of a self-synchronizing slip clutchoperated by the controller, a gear coupling and/or a torque converter.

The solar radiation can be applied directly by concentrating thereflections from an array of heliostats, directly on the flow,particularly the exhaust of a gas turbine. In one embodiment, theexhaust is passed through a solar concentrator on which concentratedsolar energy is reflected by lenses and/or mirrors. The additionalturbine is configured to expand steam that has been boosted in pressureand/or temperature by the solar collector, especially to a supercriticalphase state. The solar collector can include tracking movable reflectorsconfigured to concentrate the solar radiation at the zone where solarradiation is thermally coupled to the flow of exhaust gases and/or to aonce-through boiler for steam.

Where the solar collector boosts the steam pressure and temperature to asupercritical level, it is advantageous to employ a steam turbine thatis optimized to expand supercritical steam. In one embodiment, thepressure is at least 3,750 psia and the temperature is at least 600° C.(1,112° F.). In another embodiment, the gas turbine produces a nominaloutput between about 585 and 700° C. (1,000 to 1,200° F.) and the solarcollector is configured to increase the temperature up to 920° C.(1,600° F.) at maximum solar radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

For purposes of illustration, nonlimiting examples of certainembodiments according to the invention are described below and are shownin the drawings, wherein:

FIG. 1 is a schematic depiction of an ISCC power plant having a GTCCpart and a CSP part comprising a solar field and solar steam generator,illustrated generically.

FIG. 2 is a schematic depiction of an exemplary embodiment of thepresent invention, characterized by a configuration wherein a CSPcollector directs heat energy directly to raise the temperature in theexhaust flowpath of a gas turbine for producing supercritical steam thatis in turn passed through steam turbines that are configured for thesupercritical steam temperature, pressure and flow parameters.

FIG. 3 is a schematic diagram of a practical aspects of an embodimentshowing flowpaths and including valves that are operated by a controller(not shown) to complement changes in operational conditions such as theavailability of sunlight.

FIG. 4 is a schematic diagram showing an alternative embodiment; and,

FIG. 5 is a schematic diagram of the embodiment substantially as in FIG.4, provided with flowpaths and controls.

FIG. 6 is a schematic showing a further alternative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the elements of a GTCC power plant having an associatedsolar power collection portion. Solar radiation is collected by multiplecollector elements distributed over an area, so long as sunshine ispresent and incident on the collector elements. When solar radiation ispresent, it is used to heat a heat transfer medium used for extractingmechanical energy. Generally, the heat transfer medium produces orelevates the temperature of steam, through a heat exchanger. The flow ofsteam is passed through a steam turbine, resulting in a reduction intemperature or pressure (that is, enthalpy) in exchange for extractedmechanical energy. Typically the mechanical energy provides torque to adrive shaft coupled to an electric generator. In this disclosure, itshould be appreciated that the term “coupled” denotes an operationalconnection of two or more elements wherein the connection may be director indirect, such as a connection through intervening elements.

For purposes of efficiency, it is generally appropriate to pass the flowof steam or other flow media through a series of energy transitionstages that may result in use of the available heat energy at differentthermodynamic conditions. For example, the flow through the system maypass plural steam turbines configured for progressively lower pressuresand temperatures and progressively greater flow volumes. Re-heating flowpaths can be used to recover heat energy for useful applications. Theflow of steam can be recycled or potentially can end in a phase changefrom steam to liquid water. Likewise energy may be extracted byexploiting a temperature difference between the steam or water and alower temperature energy sink.

According to the embodiment shown in FIG. 2, rather than coupling asolar field of heat collection elements coupled by a heat transfer fluidpath to a steam generator (or other variation of a boiler), a CSP systemis configured to reflect and concentrate the solar radiation incident onthe solar field directly to the exhaust gases from a gas turbine. Thisarrangement of direct solar concentration from reflectors andconcentrated heating of the gas turbine exhaust is preferably exclusiveof use of a heat transfer fluid, although it is conceivable to providefor both techniques for solar power collection or for other combinationsof techniques exploiting solar, combustion, geothermal, wind and otherenergy sources in various configurations of combined cycle plants.

As shown in FIG. 2, when there is no solar field contribution orinsufficient solar contribution, such as at night or in overcast daytimeconditions, the system produces electric power as a standard GTCCsystem. In this arrangement, the GTCC system can comprise a horizontal,Three-Pressure Reheat (3PRH) or Two-Pressure Reheat (2PRH) HRSG with aOnce-Through Boiler (OTB) or once through evaporator section, with thesteam flowpath coupled to a standard Steam Turbine Generator (STG)configured for efficient operation at the temperature, pressure and flowconditions that are present. In one embodiment, the HRSG generates highpressure (HP) steam at subcritical conditions, e.g., 1,800 psia and1,050° F.

The gas turbine can be an advanced F, G, H or J class unit from originalequipment manufacturers such as General Electric (e.g., Frame 7 or 9),Siemens (e.g., SGT6-8000H or SGT5-8000H), Alstom (e.g., GT24 or GT26) orMitsubishi (e.g., 501J or 701J). These units burn combustion fuels andprovide torque as well as a high temperature exhaust gas flow.

The HRSG has a “once-through” HP section. An example is the Bensontechnology of Siemens. The remaining sections can have a conventionaldrum-type design.

The steam turbine can be an advanced unit capable of high pressures andtemperatures at the upstream steam flow path, progressively lowerpressure turbine paths to accommodate the expanding steam and endingwith multiple-flow LP turbine with long last stage buckets (LSB)commensurate with low condenser pressures. Examples are GeneralElectric's D series, Alstom's STF series or Siemens SST series units.Each element in the series is configured for extracting energy at aprogressively lower energy state of the steam.

The solar field can comprise a plurality of reflectors (e.g., severalthousand) called heliostats. The heliostats focus incident solarradiation on a “beam-down” reflector at the top of a tall tower asdescribed in U.S. Pat. No. 5,578,140-Yogev et al., hereby incorporatedby reference. The beam-down reflector applies the concentrated solarenergy to the exhaust path of the gas turbine.

The solar concentrator can be based on volumetric receiver technologywhere the concentrated radiation is absorbed within a porous structure.Prototypes of volumetric air receivers have been developed and tested ona limited scale (a few thermal megawatts), e.g., Jülich, Germany. See,Koll, G., et al., “The Solar Tower Julich—A Research and DemonstrationPlant for Central Receiver Systems.”

The technology using such volumetric receivers is primarily based onheating gas turbine combustion air, which is also known “Solar GasTurbine” or “Solar Hybrid Gas Turbine”. Yet another configuration usingsuch volumetric receivers concerns mixing gas turbine exhaust gas (525°C.) with air that has been heated in a volumetric receiver atop a tower(“solar air tower”). For example if air heated in the tower to reaches680° C., the mixture with 525° C. gas turbine exhaust may produce afinal mixed temperature of 615° C. See, Horn et al., 2004, “Economicanalysis of integrated solar combined cycle power plants; A sample case:The economic feasibility of an ISCCS power plant in Egypt,” Energy, 29,pp. 935-945.

Such a volumetric receiver is only one method of heating a gaseous fluidby concentrated solar rays. Other devices are conceivable to achievesolar concentrator heating of a gas flow as in the current invention, inparticular to increase the temperature of the exhaust flow of a gasturbine.

According to the embodiment of FIG. 2, the invention comprises twodistinct components, namely (1) a solar concentrator between the gasturbine exhaust and the heat recovery steam generator, and (2) asupercritical high pressure turbine.

The solar concentrator is placed directly into the path of the gasturbine exhaust gas. The solar concentrator receives the solar raysreflected from tower-mounted “beam-down” parabolic mirror to furtherheat the exhaust gas, for example from about 1150° F. (620° C.) up to1,600° F. (870° C.). The Supercritical High Pressure Turbine (SCHPT) canbe coupled to the main STG via a Synchro-Self-Shifting (SSS) clutch anda gearbox (or torque converter). The SCHPT generates mechanical power byexpanding the supercritical steam (typically at least 3,750 psia and1,112° F. (600° C.)) generated in the HRSG to pressures and temperaturessuitable to the configuration of the main STG. By providing theself-synchronizing clutch and gearbox coupling, the SCHPT can bemechanically disengaged when solar radiation is absent or unavailable insome predetermined minimum amplitude to justify operation of the SCHPT.When operating, the exhaust from the SCHPT can be coupled through tofurther staged turbines and other energy extraction mechanisms thatoperate at lower pressures and temperatures, and higher volume flowrates.

According to the alternative embodiments shown, the invention isconfigurable or can be controllable to operate when one or the other ofthe two distinct components are not available, namely the solarconcentrator to heat the gas turbine exhaust gas, or the supercriticalhigh pressure turbine. In one alternative embodiment, shown in FIGS. 4and 5, instead of a tower-mounted “beam-down” type reflector and solarconcentrator, a central solar receiver system is utilized to transfersolar thermal energy to the GTCC system via additional HP steamgeneration either directly or using a heat transfer fluid (the standardISCC configuration). However the additional solar thermal energy isapplied to produce supercritical steam as described above and energy isextracted using the supercritical steam turbine configuration that isdecoupleable in the absence of solar radiation.

In another alternative embodiment shown in FIG. 6, additional steamheating by “solar firing” is accommodated by the main STG. There is noseparate, clutched SCHPT as described above. This embodiment isanalogous to HRSG supplementary or duct firing where fuel burners arereplaced or supplemented by the solar concentrator.

FIG. 2 shows a partly schematic depiction of an exemplary embodimentthat employs the full complement of solar collector and concentratorwith direct heating, plus a supercritical HP steam turbine that iscoupled to contribute energy when solar energy is available and isdecoupleable when solar energy is unavailable or insufficient. FIG. 3 isthe flow diagram associated with FIG. 2.

Referring to FIGS. 2 and 3, in hybrid solar-thermal operation mode, thesolar concentrator can heat the gas turbine exhaust gas up to about1,600° F. (870° C.) by application of the concentrated solar beam fromthe solar field onto the gas turbine exhaust. Inasmuch as the flow ofexhaust gases is necessarily confined, a volumetric receiver technologyis appropriate where the concentrated radiation is absorbed within aporous structure through which the exhaust gases flow. High pressuresteam at 3,750+ psia and 1,112° F. (600° C.) is generated in the OTB andsent to the SCHPT. The SCHPT is connected mechanically to the main STGvia a SSS clutch. The SSS clutch engages mechanically when the SCHPT isoperating (hybrid mode) and disengages when the SCHPT is not operating(thermal mode). This equipment and operating mode ofengagement/disengagement is based on shaft speed matching (hence theterm “synchronizing” in the name or designation SSS). Synchronizingcouplings of this type are commonly utilized in single-drive-shaft GTCCsystems. Depending on the nominal speeds of the SCHPT and the driveshaft, a speed-reduction gearbox or torque converter also can couple theSCHPT and the main STG as described more particularly below.

In fundamental engineering terms, the impact of the invention is asubstantial increase in the mean-effective heat addition temperature,METH, which is the logarithmic average of the GT exhaust gas temperatureat the inlet and exit of the solar concentrator. In numerical terms,consider that METH for the current technology shown in FIG. 1 is at itshighest with HP steam generation around 650° F. With the currentinvention, with 1,150° F. at the solar concentrator inlet (for a typicaladvanced F class gas turbine) and 1,600° F. at the solar concentratorexit, METH is more than 1,300° F. Such a dramatic increase in METH andthe commensurate increase in maximum power denoted by E_IN open the doorto maximizing ISCC potential. This is the theoretical foundation of thecurrent invention.

Merely increasing METH would waste exergy (i.e., useful work generationpotential quantified by E_IN) if the flow at increased temperature waspassed through a steam generator that has been configured for otherconditions, such as the standard lower pressure and temperatureconditions for which the main steam turbine was presumably optimized.Currently, the best available (and proven) technology for Rankine cyclesteam power generation is represented by supercritical steam turbines,which are deployed in fossil power plants with coal-fired boilers. Inthe most recent and advanced versions of those plants, steam isgenerated at pressures up to 4,350 psia (300 bar) or higher attemperatures up to 1,112° F. (600° C.).

The first advanced supercritical demonstration plant, Eddystone in USA,was designed in the late 1950s and had a maximum steam temperature of650° C. (1,200° F.). Due to commercial reasons, steam temperaturesstayed around 550° C. through the 1970s and 1980s. Recently,environmental considerations associated with carbon dioxide emissionsand the greenhouse gas effect, and other factors, have forced a changeto higher temperatures. Ultra-supercritical steam technology withpressures up to 4,700 psia (325 bar) and 630° C. steam (1,166° F.) maybecome the commercial state-of-the-art within the next two decadesassuming some progress with available materials (e.g., mixtures offerritic and austenitic steels). Research is underway to reach 700° C.steam temperatures with Ni-based alloys (e.g., European Community'sTHERMIE 700 project), which presents significant challenges in componentdesign (specifically high-pressure steam valve chests and turbinecasings with thick walls).

According to the present invention, gas turbine exhaust gas is heated to1,600° F. in the solar receiver for production of supercritical steam ina once-through HP boiler section in the HRSG with pressures of 3,750+psia and temperatures up to 600° C. (1,112° F.) in the SCHPT. In fact,supercritical steam cycles have been considered for Rankine bottomingcycles of GTCC plants. One such system is described in U.S. Pat. No.7,874,162-Tomlinson et al., hereby incorporated by reference.

At supercritical steam conditions, the density of motive (high pressure)steam is very high. Consequently the specific volume, v=1/ρ, and thevolumetric flow rate are lower than typically encountered in standard3PRH subcritical systems (e.g., as much as 50% lower for the same steammass flow rate). In the HRSG of a combined cycle power plant, steamgeneration is limited by the GT exhaust energy. For a 1×1 GT-HRSG train,even with the larger advanced F class machines, supercritical steamgeneration is typically not at a level commensurate with therequirements of large supercritical steam turbines. For a feasibledesign with reasonable bucket height and stage efficiencies, the turbinerotational speed should be increased to above 3,000-3,600 rpm for directelectric generation at 50 and 60 Hz, respectively.

Steam turbine stage efficiency is strongly dependent on volumetric flow,{dot over (V)}, e.g.

$\eta \propto \frac{\overset{.}{V}}{U \cdot r^{2}}$

where r is the mean diameter of the turbine bucket and U is therotational speed at r, which is given by

U = ω ⋅ r $\omega = \frac{2\; {\pi \cdot N}}{60}$

where ω is the angular speed and N is the number of revolutions of theturbo-generator shaft (3,000 or 3,600 rpm). Hence, combining theformulas above

$\eta \propto \frac{\overset{.}{V}}{N \cdot r^{3}}$

Since {dot over (V)} is proportional to the product of r and bladeheight, h, a reduction in the “swallowing capacity” of the STcommensurate with the reduction in {dot over (V)} can be achieved by areduction in r and/or h at the same proportion. Shorter blades aredetrimental to ST performance due to magnified leakage and profilelosses. For a 50% reduction in volumetric flow, without an unfavorablereduction bucket heights, maintaining the same level efficiency can beachieved by decreasing the mean diameter by about one-third and doublingthe shaft speed, e.g., to 7,200 rpm for a 60 Hz system. This results ina smaller ST circumference (blade-hub-blade) than a comparable 3,600 rpmunit. The smaller, high-speed, high-efficiency supercritical turbine isconnected to the main STG shaft, which is rotating at thegrid-determined rate of 3,600 rpm, via a speed reduction (2:1 ratio)gearbox. The point is that the structural configuration and operationalparameters of a turbine such as a steam turbine generator are closelyassociated with the temperature and pressure of the steam or other gasthat is flowing through the turbine. An aspect of the invention is thatprovisions are made for efficient operations when using available solarenergy to boost the gas turbine exhaust temperature sufficiently toproduce supercritical steam, because the provisions include a steamturbine generator that is optimally configured for supercritical steampressure and temperature conditions. That steam turbine generator isselectively coupleable mechanically to the main power generation train,comprising the main steam turbine and its generator, when solarradiation is available. The main steam turbine generator is optimizedfor steam at lower temperature and pressure, namely for the steamconditions that are present when solar radiation is not available. Thesecan also be the conditions present at the output from the high pressureturbine, where the temperature and pressure have been reduced to belowcritical conditions. In this way, the high pressure and main steamturbines can be staged to extract energy from the steam at progressivelylower temperatures and higher volume flow rates.

According to the present invention, the SCHPT is only operational whensolar energy is generating supercritical HP steam in the HRSG. Since thesolar energy injection per the present invention is analogous tosupplementary firing (without the concomitant fuel consumption), gasenergy is sufficient to generate supercritical steam at high pressureand temperature in adequate quantities. The SCHPT is connected to themain STG via the SSS clutch, which engages when the SCHPT speed (with orwithout a speed reduction gearbox) reaches the main steam turbinegenerator speed. In this way, solar energy contributes to powergeneration when the amount of solar energy is at least sufficient forthe SCHPT to come up to the nominal speed of the main STG

In a multiple GT configuration (i.e., 2×1 or 3×1, meaning 2 or 3 GT-HRSGtrains) steam production in the HRSGs can be high enough to facilitate afeasible SCHPT design with 3,000 or 3,600 rpm nominal speed (i.e., thesame nominal speed as the main STG, depending on electric power gridfrequency), in which case the speed-reduction gearbox can be eliminated.The connection between the SCHPT and the main STG in that case is viathe SSS clutch only.

FIG. 3 illustrates the invention with reference numerals for theelements identified as follows. The solar concentrator (2) is locatedbetween the GT (1) and HRSG (3). Exhaust flow (4) from the GT enters theconcentrator and heated by the concentrated solar beam (7) from thesolar field (6) to a very high temperature. Heated exhaust gas (5)enters the HRSG (3) and generates steam at a pressure and temperatureabove the critical point of the H₂O in the steam substance.Supercritical high pressure steam (23) is coupled to the SCHPT (8) via athrottle valve (18). When SCHPT (8) is operational and turning on itsshaft support (11), it is mechanically connected to the shaft of themain STG (12) via a SSS clutch (9) and, if necessary, a speed-reductiongearbox (10).

Supercritical steam expands as it passes through the SCHPT (8),producing torque, and is exhausted to the HP section (13) of the mainSTG (12). Thereafter, the process is the same as in a conventional GTCCsteam turbine. Steam expands through the intermediate pressure IPsection (14) and the LP section (15) in a staged manner, and isexhausted to the steam condenser (17). Shaft power from steam turbines(8) and (12) is converted to electric power in the generator (16)connected to the STG (12). When there is no solar energy available(e.g., at night), exhaust gas streams (4) and (5) are at the sametemperature. Steam generated in the HRSG is subcritical. Thus, the SCHPTthrottle valve (18) is closed and the main STG HP throttle valve (19) isopened. The SCHPT (8) slows down and the SSS clutch (9) disengages sothat no power is contributed by the SCHPT. During this process, thebypass valve (20) can be opened to slow down the SCHPT and divert steamto the HP section (13) of the main STG (12) in a controlled manner.During some operational conditions (primarily during startup or shutdownor at low loads) part or all of the SCHPT exhaust can be diverted to theIP section (14) of the main STG (12) via the exhaust valve (21). Theexhaust valve (22) can be partially or fully closed as needed toaccomplish the transition and operation in a controlled manner.

FIG. 4 illustrates an alternative embodiment wherein integration of CSPand GTCC functions are achieved with additional high pressure steamgeneration in the receiver via direct steam generation DGS or with asolar steam generator SGS (for example in a molten salt based system).The clutch-coupled SCHPT of the invention as described introduces keyadvantages, which differ from ISCC systems that are known. Theadvantageous differences include an ability to run the GTCC bottomingcycle at advanced steam cycle conditions, for example 2,200 to 2,400psia and 1,100° F. steam at the steam turbine throttle without anycontribution from concentrated solar power (i.e., no power boost); andalso, an ability to go to very high (supercritical) steam pressures withCSP contribution for significantly improved overall efficiency of theISCC facility (with and without CSP power boost).

In order to maximize solar contribution with the standard ISCC, it maybe necessary to significantly degrade the non-augmented steam cycle,e.g., to pressures of 1,600 psia or even lower, say, 1,400 psia.According to the invention, as explained above, this is not necessary.However, this approach might be feasible within certain economicconstraints. Even in that case, the invention per the alternativeembodiment in FIG. 4 is advantageous. For example, the main STG can bedesigned for 1,400 psia and 1,000° F. throttle steam conditions foroperation with no solar steam generation. The separate HP turbine can bedesigned for 2,500 psia and 1,100° F. throttle steam conditions toaccommodate additional steam flow and temperature with solar steamgeneration. In exactly the same manner as the SCHPT, the subcritical HPturbine is decoupled from the main STG via the SSS clutch when there isno solar steam generation. This will ensure optimal ST performance whenoperating in hybrid and thermal modes.

Referring to FIG. 5, the solar receiver and/or solar steam generator isa part of the CSP plant (30). Exhaust flow from the GT (4) enters theHRSG (3) and generates steam at high pressure and temperature. Exemplaryconditions are about 2,400 psia and 1,100° F. at the steam turbineinlet/throttle.

In solar-augmented mode, high pressure feed water (31) is provided tothe concentrated solar power CSP plant (30) and high pressure steam (32)is returned to the HRSG (3). At sufficiently high solar thermal input,the mass flow rate of additional high pressure steam increases to alevel to facilitate HRSG steam generation above the critical point ofthe H₂O substance. The critical point is generally the temperature andpressure at which the steam changes from separate gas and liquid phasesinto a continuous plasma-like phase.

The HRSG (3) advantageously is of a once-through design configured togenerate steam at subcritical or supercritical temperatures/pressures.The main STG (12) need not be configured to withstand such pressures.Supercritical HP steam (23) is sent to the SCHPT (8) via the throttlevalve (18). The SCHPT is configured for supercritical steam at highpressure, namely to expand steam from a supercritical pressure to apressure commensurate with the design of the main STG (12). When theSCHPT (8) is operational, it is connected to the shaft of the main STG(12) via the SSS clutch (9) and, according to some embodiments, with aspeed-reduction gearbox (10). The rest of the system is substantiallythe same as described with respect to FIG. 3.

The alternative embodiment of FIG. 6 can be envisioned as astraightforward supplementary or duct-fired HRSG having a volumetricreceiver configured for absorbing solar heat and conveying the heat intothe gas turbine exhaust. The amount of solar thermal input is limited inpart by the swallowing capacity at the inlet of the steam turbine, alonga steam conduit coupled in heat exchange relationship with thevolumetric receiver.

Some of the benefits of the foregoing exemplary and alternativeembodiments can be appreciated by considering and comparing parametersnumerically. A baseline for comparison is a standard GTCC in a 2×1configuration. The gas turbines can be advanced General Electric Frame 7units. The steam turbine can be a state-of-the-art General Electric Dclass unit discharging to an air-cooled condenser. The steam cycle is1,800 psig and 1,050° F./1,050° F. for main and hot reheat steam. Thesteam turbine maximum pressure is 2,500 psig. This unit is nominallyrated as a 600 MW—56% net (ISO base load) machine. On a hot day (85° F.and 50% relative humidity), as is typical of GTCC power plants, theoutput drops by about 5% (roughly 30 MW) due to a reduction in gasturbine airflow.

Taking the hot day performance of the GTCC as the basis for comparison,the invention in its preferred and alternate embodiments is applied tosolar power augmentation. A comparison can be made between solar poweraugmentation as described versus supplementary firing (a standard methodof hot day power augmentation in addition to gas turbine inlet airchilling) and standard ISCC operation with high pressure steamgeneration in the solar steam generator of the central receiver system.The results are summarized in Table 1, which demonstrates substantiallyimproved solar thermal energy utilization at a higher efficiency (bothincremental Rankine cycle and net GTCC thermal efficiencies) vis-à-visthe standard ISCC. Specifically, with a base GTCC of 56% efficiency,64.5% is possible in ISCC mode with the current invention with only1,450° F. gas temperature at the exit of the solar concentrator. Thisresult indicates that utilizing a state-of-the-art H or J class gasturbine with 60% base efficiency in GTCC mode, 68.5% is possible inISCC. Similarly, with 1,600° F. gas temperature at the exit of the solarconcentrator, with an H or J class GTCC, 70% efficiency is possible inISCC mode. Careful system optimization is likely to take the ISCCperformance beyond these figures.

TABLE 1 GTCC ISCC Solar Fired Fired Unfired FIG. 2 FIG. 4 FIG. 6 AmbientHot Hot Hot Hot Hot Hot Solar Thermal Input MWth NA 150 217 217 173 169Solar Fraction 0.14 0.21 0.21 0.16 0.16 Solar Concentrator F. NA Sat.1,450 1,450 1,050 1,380 Exit SC HPT Throttle P psia NA NA 3,750 4,2503,750 NA SC HPT Throttle T F. NA NA 1,111 1,111 1,022 NA SC HPT OutputkW NA NA 18,561 24,707 18,172 NA Δ CC Output MWe 73 50 84 85 70 62Incremental Rankine 33.4% 38.5% 39.3% 40.8% 36.5% Cycle Efficiency Δ NetCC Efficiency % −2.59 4.74 8.32 8.48 6.69 6.22 Condenser Pressure in. Hg4.48 4.32 4.56 4.54 4.32 4.30 Main STG Throttle P psia 2,515 2,515 2,5332,489 2,482 2,515 Main STG Throttle T F. 1,037 854 989 944 898 1,110Main STG Reheat T F. 1,048 904 1,048 1,048 975 1,048

As provided herein, a gas turbine combined cycle apparatus includes acombustion turbine (another term for gas turbine) exhausting hotcombustion gas products to a heat recovery system with at least onesteam generator. Generated steam is expanded in a main steam turbinecoupled to an electric generator for generating electric power. A gasheater is coupled along the flow path between the combustion turbine andthe heat recovery system. The gas heater applies concentrated solarenergy from a solar field to add heat to at least a portion of the hotcombustion gas products from the combustion turbine.

The heat recovery system can have at least one once-through evaporatoror boiler section configured to increase the energy of steam from thetemperature obtained using only combustion gas products in the absenceof solar radiation, to supercritical conditions when solar radiation isavailable. It is also possible to include at least one additionalevaporator section configured to increase the energy of the steam tosubcritical pressure levels. Plural steam turbines can be deployed orare deployable to extract mechanical energy from flows of steam atdifferent temperature, pressure and flow conditions, at least one suchflow is boosted in temperature and/or pressure by the solar radiation.The respective flows at different energy states can be staged throughsuccessive turbines that each operate to expand and extract energy atsuccessively lower pressures and higher volume flow rates. A controllerand associated valves responsive to the controller are configured toroute steam carrying available energy to selected ones of the pluralsteam turbines or between appropriate stages. At least one controllableclutch responsive to the controller can selectively engage to anddisengage from a shaft mechanically coupled, directly or indirectly, toan electric generator at least one of the plural steam turbines.

The invention also encompasses a solar concentrator section incombination with power generation plant apparatus defining a flow pathfor gaseous products where at least one energy extraction deviceexploits the gaseous products to product useful work. The solarconcentrator has a conduit with inlet and outlet couplings configured toengage with the source and the energy extraction device. The conduitincludes or is coupled to a heat transfer section operable to receiveand transfer incident radiation to the gas flowing through the conduit.

According to the embodiments disclosed herein, the solar concentrator isconfigured to direct solar radiation, which is available intermittentlyand at varying intensity, directly into the hot flow medium (such as thegas turbine exhaust) that carries heat into the heat recovery steamgenerator (HRSG). It is possible, for example, to increase the exhausttemperature up to 1,600° F. (870° C.), leading to elevation of thetemperature and pressure of the steam produced from the HRSG for drivingsteam turbines that apply shaft work (torque) to an electricalgenerator.

The HRSG is employed to produce steam for driving the steam turbineswhether or not the solar heat energy is available at any given time. Inthe absence of solar radiation, the steam from the HRSG is preferablyrouted through a series of high, intermediate and low pressure turbinesconfigured and optimized to extract the available energy efficientlyfrom expanding the steam. When solar energy is available, thetemperature and pressure of the steam from the HRSG becomes elevated intemperature and pressure above their levels in the absence of the solarradiation. It is an aspect of the invention that a steam turbine stageoptimized for elevated pressure and temperature parameters achieved viasolar radiation is selectively deployed by being coupled into thebeginning (the high temperature/pressure end) of the steam path throughthe staged series of steam turbines when solar radiation is available,and taken out of when solar radiation is unavailable.

In an advantageous embodiment, the solar concentration and HRSG steamgeneration capacities are arranged such that when the added energy ofsolar radiation is available, the resulting elevated temperature andpressure at the HRSG can be such that the steam becomes supercritical(such as 3,750 psia and 1,100° F. or more). When solar radiation isunavailable, a subcritical steam energy level (e.g., 1,800 psia and1,050° F.) may ensue. Advantageously, the steam turbine that is deployedand coupled into the steam flow path at the high energy end is optimizedfor expanding supercritical steam through that pressure difference,contributing torque to drive the generator, and passing alongsubcritical steam into the staged series of turbines at an energy levelthat is comparable to the level when solar energy is not available.

The selective insertion of a mechanically engageable and disengageablesteam turbine stage at the high energy end of a steam expansion path toexpand periodically available high energy steam, is particularly usefulfor solar energy, which is intermittent by its nature. The technique isadvantageous where the higher energy steam and selectively deployablesteam turbine are supercritical whereas the turbines otherwise deployedfor lower energy steam are subcritical. In that situation, the higherenergy steam turbine can be configured specifically for supercriticalparameters, leading to improved efficiency in the extraction ofavailable energy. The technique improves operability and efficiency ifoperations with or without the intermittent additional energy source issufficient to produce a distinct phase. That is, the HRSG steam may besupercritical or subcritical when solar energy is available or not, asopposed to changing from subcritical to supercritical with the addedsolar energy, provided that the selectively inserted steam turbine stageis configured for the higher temperature steam. In one embodiment, anHRSG may produce steam, for example, at 1,400 psia in the absence ofsolar energy, and generate higher pressure but still subcritical steamwhen solar energy is present, e.g., steam at 2,500 psia. The selectivelydeployed steam turbine in that case is optimized to expand the 2,500psia steam to 1,400 psia and to feed steam into a next steam turbinestage optimized to expand the steam from 1,400 psia and so on.

The invention has been disclosed in connection with several exemplaryembodiments and alternative embodiments; however the invention can beembodied in other specific ways as will now be apparent to personsskilled in the art. Reference should be made to the appended claimsinstead of the foregoing discussion of embodiments and examples, toassess the scope of the invention in which exclusive rights are claimed.

What is claimed is:
 1. An apparatus for generating mechanical work fromthermal energy, comprising: at least a first power generation componentoperable to produce at least one flow medium with an elevated energystate from which energy can be extracted; at least one energy extractioncomponent operable to extract mechanical work from the flow medium,extraction of such energy reducing the flow medium to a less elevatedenergy state; a flowpath defined for the flow medium from the firstpower generation component to the energy extraction component; a solarconcentrator operable when solar radiation is available to direct solarheat energy into the flow medium along the flowpath, thereby increasingthe elevated energy state of the flow medium.
 2. An electric powergeneration system comprising: at least one steam generator coupled in arecirculating flow path to supply a production flow of steam at nominalproduction pressure and temperature to a main steam turbine, the mainsteam turbine being mechanically coupled to an electric generator forgeneration of electric power by extracting energy from expansion of theproduction flow of steam; wherein at least one additional steam turbineis coupled to the recirculating flow path and operated at a secondproduction pressure and temperature that are one of a higher energystate than the nominal production pressure and temperature; a solarcollector configured to apply concentrated solar radiation to at leastone zone along the recirculating flow path during periods of sufficientavailable solar energy, the solar collector increasing a pressure andtemperature at said zone to the second production pressure andtemperature; and, a controller coupled to sensors assessing thecondition of steam from the recirculating flow path and operating atleast one valve for selectively deploying the additional steam turbine.3. The electric power generation system of claim 2, wherein theadditional steam turbine is mechanically coupled to contribute torque toturn the electric generator, and further comprising a synchronizingclutch operated by the controller.
 4. The electric power generationsystem of claim 3, wherein the additional turbine is configured tooperate at a said second production pressure and temperature that arehigher than the nominal production pressure and temperature, and whereinan exhaust from the additional turbine is coupled into the recirculatingflow path upstream from the main steam turbine.
 5. The electric powergeneration system of claim 2, wherein the additional turbine isconfigured to expand steam boosted in pressure and temperature by thesolar collector.
 6. The electric power generation system of claim 5,wherein the solar collector comprises an array of movable reflectorsconfigured to concentrate the solar radiation at said zone.
 7. Theelectric power generation system of claim 6, wherein the concentratedsolar radiation boosts the pressure and temperature of the steam to asupercritical level.
 8. The electric power generation system of claim 7,wherein the pressure is at least 3,750 psia and the temperature is atleast 600° C. (1,112° F.).
 9. The electric power generation system ofclaim 7, wherein the zone at which the solar collector concentrates thesolar radiation is along an exhaust path of a gas turbine in the flowpath.
 10. The electric power generation system of claim 9, wherein thegas turbine produces a nominal exhaust flow at about 585 to 700° C.(1,000 to 1,200° F.) and the solar collector is configured to increasethe temperature up to about 920° C. (1,600° F.) at maximum solarradiation.
 11. A gas turbine combined cycle apparatus comprising: acombustion turbine exhausting hot combustion gas products along a gasflow path to a heat recovery system having at least one steam generatora main steam turbine coupled to an electric generator for generatingelectric power; a gas heater coupled along the gas flow path, whereinthe gas heater applies concentrated solar energy from a solar field toadd heat to at least a portion of the hot combustion gas products fromthe combustion turbine.
 12. The gas turbine combined cycle apparatus ofclaim 11, wherein the heat recovery system comprises at least oneonce-through evaporator section configured to increase the energy ofsteam to supercritical conditions.
 13. The gas turbine combined cycleapparatus of claim 12, further comprising at least one additionalevaporator section configured to increase the energy of the steam to asubcritical pressure levels.
 14. The gas turbine combined cycleapparatus of claim 11, comprising plural steam turbines operable toextract mechanical energy from steam at different temperature, pressureand flow conditions, and further comprising a controller and associatedvalves responsive to the controller, configured to route steam carryingavailable energy to selected ones of the plural steam turbines.
 15. Thegas turbine combined cycle apparatus of claim 14, further comprising atleast one controllable clutch responsive to the controller forselectively mechanically engaging and disengaging a shaft between theelectric generator and at least one of the plural steam turbines.
 16. Aheat transfer section configured to define a flow path in a powergeneration apparatus having at least one source of gaseous products andat least one energy extraction device producing useful work from thegaseous products, comprising: a conduit with inlet and outlet couplingsconfigured to engage with the source and the energy extraction device;wherein the conduit comprises a heat transfer section operable toreceive and transfer incident solar radiation to gases flowing throughthe conduit; a solar energy collector and concentrator system havingreflectors arranged to direct solar radiation onto the heat transfersection.